U.S. patent number 4,444,711 [Application Number 06/333,426] was granted by the patent office on 1984-04-24 for method of operating a two-shot injection-molding machine.
This patent grant is currently assigned to Husky Injection Molding Systems Ltd.. Invention is credited to Robert D. Schad.
United States Patent |
4,444,711 |
Schad |
April 24, 1984 |
Method of operating a two-shot injection-molding machine
Abstract
There is disclosed a method of operating an injection-molding
machine to produce a composite item that includes a portion of soft
material such as a set of bristles tending to cling to a mold
cavity in which it is formed. At a first station, a relatively
flexible plastic material is injected into a first cavity shaped to
define the portion of soft material of the composite item, that
portion including a backing layer. The portion of soft material is
then allowed to cool, and at a subsequent station there is injected
a relatively rigid plastic material into a second cavity bounded by
the backing layer. The second cavity defines a harder portion of
the composite item such as a brush handle. After the latter portion
has also cooled, the portion of soft material is separated from the
first cavity by pulling on the harder portion at a rate of speed
slow enough to avoid rupture of the portion of soft material.
Inventors: |
Schad; Robert D. (Toronto,
CA) |
Assignee: |
Husky Injection Molding Systems
Ltd. (Bolton, CA)
|
Family
ID: |
23302725 |
Appl.
No.: |
06/333,426 |
Filed: |
December 21, 1981 |
Current U.S.
Class: |
264/243; 264/255;
264/334 |
Current CPC
Class: |
B29C
45/045 (20130101); B29C 45/1628 (20130101); B29C
45/2626 (20130101); B29C 45/1676 (20130101); B29C
45/1657 (20130101); B29L 2031/425 (20130101); B29L
2031/42 (20130101) |
Current International
Class: |
B29C
45/16 (20060101); B29C 45/26 (20060101); B29F
001/14 (); B29C 005/08 () |
Field of
Search: |
;264/243,255,334,250
;425/805,576 ;15/186,187,188 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Hoag; Willard E.
Attorney, Agent or Firm: Ross; Karl F.
Claims
I claim:
1. A method of operating a two-shot injection-molding machine to
produce composite articles each having one part of a relatively
flexible modified rubber with a tendency to cling to a mold cavity
during an extended hardening period and further having another part
of a relatively rigid and more rapidly hardening plastic material
adjoining said one part, comprising the steps of:
(a) injecting said modified rubber into a first cavity formed at
one station of the machine between a primary mold portion and a
secondary mold portion juxtaposed with each other, thereby
producing a first workpiece section;
(b) retaining said first workpiece section on said primary mold
portion while removing said secondary mold portion therefrom to
expose a major surface of said first workpiece section while
letting same cool;
(c) juxtaposing a tertiary mold portion at another station of the
machine with said primary mold portion to form a second cavity
bounded by said major surface of the first workpiece section;
(d) injecting said relatively rigid plastic material into said
second cavity to produce a second workpiece section adhering to
said first workpiece section along said major surface;
(e) allowing said second workpiece section to cool with further
cooling of said first workpiece section;
(f) thereafter separating said tertiary mold portion from said
primary mold portion; and
(g) gradually pulling said second workpiece section away from said
primary mold portion at a controlled rate slow enough to maintain
the coherence of the entire workpiece.
2. A method as defined in claim 1 wherein step (g) is accompanied
by a gradual separation of two members of said primary mold portion
jointly defining part of said first cavity for partly detaching
said first workpiece section from the corresponding cavity
walls.
3. A method as defined in claim 1 or 2 wherein step (g) is
performed concurrently with step (f) with retention of the
workpiece on said tertiary mold portion, followed by a dislodgment
of said workpiece from said tertiary mold portion.
4. A method as defined in claim 1 or 2 wherein step (g) is
performed after a further cooling period following separation of
said tertiary mold portion from said primary mold portion in step
(f).
5. A method of operating a two-shot injection-molding machine to
produce composite brushes each having a set of bristles of a
relatively flexible modified rubber with a tendency to cling to a
mold cavity during an extended hardening period and further having
a handle of a relatively rigid and more rapidly hardening plastic
material joined to said set of bristles, comprising the steps
of:
(a) injecting said modified rubber into a first cavity formed at
one station of the machine between a primary mold portion and a
secondary mold portion juxtaposed with each other, said first
cavity including a multiplicity of elongate cells in said primary
mold portion and an adjoining clearance in said secondary mold
portion to produce a set of bristles integral with a common backing
layer;
(b) retaining said bristles in said cells on said primary mold
portion while removing said secondary mold portion therefrom to
expose a major surface of said backing layer while letting same
cool together with said bristles;
(c) juxtaposing a tertiary mold portion at another station of the
machine with said primary mold portion to form a second cavity
bounded by said major surface of the backing layer;
(d) injecting said relatively rigid plastic material into said
second cavity to produce a handle adhering to said backing layer
along said major surface;
(e) allowing said handle to cool with further cooling of said
backing layer and said bristles;
(f) thereafter separating said tertiary mold portion from said
primary mold portion; and
(g) gradually pulling said handle away from said primary mold
portion at a controlled rate slow enough to maintain the coherence
of the entire brush so molded.
6. A method as defined in claim 5 wherein step (g) is accompanied
by a gradual separation of two members of said primary mold portion
jointly defining said cells for partly detaching said bristles from
the cell walls.
7. A method as defined in claim 5 or 6 wherein step (g) is
performed concurrently with step (f) with retention of the brush on
said tertiary mold portion, followed by a dislodgment of the brush
from said tertiary mold portion.
8. A method as defined in claim 5 or 6 wherein step (g) is
performed after a further cooling period following separation of
said tertiary mold portion from said primary mold portion in step
(f).
9. A method of operating a two-shot injection-molding machine to
produce composite articles each having one part of a relatively
flexible modified rubber with a tendency to cling to a mold cavity
during an extended hardening period and further having another part
of a relatively rigid and more rapidly hardening plastic material
adjoining said one part, the machine being provided with a
multi-sided turret sequentially rotatable through a plurality of
angular positions between two platens which are relatively
displaceable in each turret position within a respective molding
cycle between a mold-open position and a mold-closed position,
comprising the steps of:
(a) injecting said modified rubber into a first cavity formed in a
first angular turret position between a primary mold portion on a
side of said turret and a secondary mold portion on one of said
platens juxtaposed therewith in a mold-closed position of a first
molding cycle thereby producing a first workpiece section;
(b) retaining said first workpiece section on said primary mold
portion while removing said secondary mold portion therefrom in a
mold-opening stroke of said first molding cycle to expose a major
surface of said first workpiece section;
(c) letting said first workpiece section cool in a second angular
turret position during a second molding cycle concurrently with a
duplication of steps (a) and (b) on another turret side;
(d) juxtaposing a tertiary mold portion on the other of said
platens in a mold-closed position of a third molding cycle with
said primary mold portion in a third angular turret position to
form a second cavity bounded by said major surface of the first
workpiece section;
(e) injecting said relatively rigid plastic material into said
second cavity to produce a second workpiece section, adhering to
said first workpiece section along said major surface, during the
third molding cycle concurrently with a duplication of steps (a)
and (b) on a further turret side;
(f) allowing said second workpiece section to cool with further
cooling of said first workpiece section;
(g) thereafter separating said tertiary mold portion from said
primary mold portion in a mold-opening stroke of said third molding
cycle; and
(h) gradually pulling said second workpiece section away from said
primary mold portion at a controlled rate slow enough to maintain
the coherence of the entire workpiece.
10. A method as defined in claim 9 wherein step (h) is accompanied
by a gradual separation of two members of said primary mold portion
jointly defining part of said first cavity for partly detaching
said first workpiece section from the corresponding cavity
walls.
11. A method as defined in claim 9 or 10 wherein step (h) is
performed in said third angular turret position concurrently with
step (g) with retention of the workpiece on said tertiary mold
portion, followed by a dislodgment of said workpiece from said
tertiary mold portion during said third molding cycle.
12. A method as defined in claim 9 or 10 wherein step (h) is
performed in a fourth angular turret position concurrently with a
duplication of steps (a) and (b) on an additional turret side.
13. A method of operating a two-shot injection-molding machine to
produce composite brushes each having a set of bristles of a
relatively flexible modified rubber with a tendency to cling to a
mold cavity during an extended hardening period and further having
a handle of a relatively rigid and more rapidly hardening plastic
material joined to said set of bristles, the machine being provided
with a multi-sided turret sequentially rotatable through a
plurality of angular positions between two platens which are
relatively displaceable in each turret position within a respective
molding cycle between a mold-open position and a mold-closed
position, comprising the steps of:
(a) injecting said modified rubber into a first cavity formed in a
first angular turret position between a primary mold portion on a
side of said turret and a secondary mold portion on one of said
platens juxtaposed therewith in a mold-closed position of a first
molding cycle, said first cavity including a multiplicity of
elongate cells in said primary mold portion and an adjoining
clearance in said secondary mold portion to produce a set of
bristles integral with a common backing layer;
(b) retaining said bristles in said cells on said primary mold
portion while removing said secondary mold portion therefrom in a
mold-opening stroke of said first molding cycle to expose a major
surface of said backing layer;
(c) letting said bristles and said backing layer cool in a second
angular turret position during a second molding cycle concurrently
with a duplication of steps (a) and (b) on another turret side;
(d) juxtaposing a tertiary mold portion on the other of said
platens in a mold-closed position of a third molding cycle with
said primary mold portion in a third angular turret position to
form a second cavity bounded by said major surface of the backing
layer;
(e) injecting said relatively rigid plastic material into said
second cavity to produce a handle, adhering to said backing layer
along said major surface, during the third molding cycle
concurrently with a duplication of steps (a) and (b) on a further
turret side;
(f) allowing said handle to cool with further cooling of said
backing layer and bristles;
(g) thereafter separating said tertiary mold portion from said
primary mold portion in a mold-opening stroke of said third molding
cycle; and
(h) gradually pulling said handle away from said primary mold
portion at a controlled rate slow enough to maintain the coherence
of the entire brush so molded.
14. A method as defined in claim 13 wherein step (h) is accompanied
by a gradual separation of two members of said primary mold portion
jointly defining said cells for partly detaching said bristles from
the cell walls.
15. A method as defined in claim 13 or 14 wherein step (h) is
performed in a fourth angular turret position concurrently with a
duplication of steps (a) and (b) on an additional turret side.
16. A method as defined in claim 13 or 14 wherein step (h) is
performed in said third angular turret position concurrently with
step (g) with retention of the brush on said tertiary mold portion,
followed by a dislodgment of the brush from said tertiary mold
portion during said third molding cycle.
17. A method as defined in claim 16 wherein step (h) is performed
during a significant fraction of a molding cycle.
18. A method as defined in claim 17 wherein the duration of a
molding cycle is substantially 20 seconds, step (h) being performed
in substantially 3 seconds.
Description
FIELD OF THE INVENTION
My present invention relates to a method of operating an
injection-molding machine of the two-shot type for molding
composite articles partly consisting of a soft plastic material
which tends to cling to a mold cavity wherein the article is
formed. Nonlimiting examples of such articles are brushes which
have molded fine bristles of such rubbery or soft material.
BACKGROUND OF THIS INVENTION
Plastic brushes of the kind for which this invention has a
particular advantage consist essentially of two elements: the
bristles and the handle. At the present time, such brushes are
produced in conventional injection molds as integral units, the
molds including a multicell bristle cavity and an adjoining handle
cavity.
During injection, the plastic material (typically polyethylene)
fills the bristle cavity and the handle cavity. When the mold is
opened, the bristles which are attached to the handle are pulled
out of their cavities by the handle which is held in its cavity by
undercuts or by side cores. Once the bristles have been released
from these cavity cells, the brush is ejected from the handle
cavity.
The cells of the bristle cavity can be formed by simple blocks,
taper-drilled for the shape of the bristles, or composite blocks,
consisting of a pack of interfitting blades into which
bristle-shaped grooves are machined. The bristle shape is typically
triangular in cross-section and tapers toward the tip. These blade
packs can be fixed or relatively slidable, as described in my
earlier U.S. Pat. Nos. 3,004,291 and 3,128,488.
Another method of making such bristle-molding cells utilizes cavity
blocks with a multitude of round pins inserted into them. Each pin
has a plurality of axially oriented grooves in its circumference,
typically of triangular shape, whereby several bristles are formed
by each pin.
Regardless of the bristle-cavity construction which is used, it
will be readily understood that the cooling of the bristles gives
rise to problems relating to the length of time required. Because
the bristles of an ordinary brush are spaced very close to one
another, it is virtually impossible to provide cooling channels
where they are most needed, namely right next to the bristles. The
only practical way to provide any cooling is to locate the channels
around the cavity blocks and in the backing plate. This
construction results in an inefficient and therefore slow cooling
of the bristles. In fact, cooling cycles last 1/2 to 11/2 minutes,
depending upon the length and thickness of the bristles.
A serious drawback with the brushes produced from polyethylene is
the fact that, even with very fine bristles, they do not generate
the kind of lather required for such uses as "scrubbing" by medical
personnel.
A relatively new molding material, known commercially as "Kraton"
(trademark), is a modified rubber that can be processed in
conventional injection-molding equipment. As used hereinafter, the
term "modified rubber" is intended to mean a material substantially
the same as is available under the name of Kraton. This material is
more expensive than polyethylene, yet the brushes produced with it
make an excellent lather. Because Kraton is very "rubbery" (elastic
and flexible) it is an excellent material for the bristles of the
brush, but this same characteristic makes it almost impossible to
mold Kraton with the previously described methods of ejection. In
other words, if Kraton were used to make an integral brush in which
undercuts or side cores in the handle served to pull on the handle
and thus release the bristles from the cells of their cavity, the
"stickiness" of the bristles against the small-bore cells would
cause them to remain stuck and literally pull the handle out of its
own cavity, even if it were strongly engaged by undercuts or side
cores. The Kraton simply stretches and slips out uncontrollably,
leaving the bristles of the brush in associated elongate mold
cells.
This difficulty can be overcome by attaching a rigid backing to the
base of the bristles, either by fastening or by molding, which can
be used to pull the bristles out of their cells in a controlled
manner.
It should be noted that the above remarks are not limited to Kraton
or to brushes, but can be applied to any soft, rubbery material and
for any product which is difficult to remove automatically from the
cavities by conventional methods.
OBJECT OF THE INVENTION
In view of the foregoing considerations, it is the object of my
invention to provide a method of so operating a machine for molding
plastic brushes and the like that flexible and "sticky" material
such as Kraton may be used for the bristles of a brush while the
problem of effectively releasing the bristles from their cavities
is overcome.
SUMMARY OF THE INVENTION
My improved method of operating a two-shot injection-molding
machine to produce a composite article such as a plastic brush
includes several steps. There are provided in the machine a
plurality of stations at which operations can be carried out. At a
first station, a relatively flexible first plastic material is
injected into a first cavity with cells shaped to define a first
workpiece section, specifically the bristles of the brush, and with
an adjoining clearance conforming to a relatively thin backing
layer therefor. At a second station the bristles and their backing
layer are allowed to cool. At a third station a relatively rigid
second plastic material is injected into a second cavity bounded in
part by the backing layer, this cavity conforming to a second
workpiece section, specifically a handle portion for the brush. The
handle portion is allowed to cool, and then the bristles are
separated from the first cavity by gradually pulling on the handle
portion at a controlled rate slow enough to avoid rupture of the
bristles and to maintain the coherence of the resulting
workpiece.
A two-part injection-molding machine particularly suitable for the
practice of my invention includes a turret with four orthogonally
adjoining operational faces adapted to be sequentially rotated
through four angular positions in steps of 90.degree., the turret
including a primary mold portion on each face defining part of a
first cavity with a multiplicity of cells for the molding of the
bristles of the brush. At the aforementioned first station this
first cavity is completed by a secondary mold portion juxtaposed
with the primary mold portions to provide space for a relatively
thin backing layer to be molded integral with the bristles, that
station also having means for injecting the relatively flexible
first plastic material into the first cavity to form the bristles
and the backing layer. At the second station a major surface of the
backing layer is exposed to the atmosphere. A tertiary mold portion
juxtaposed with the primary mold portion at the third station forms
a second cavity bounded by the backing layer, the latter cavity
conforming to a handle portion for the brush; this station also has
means for injecting the relatively rigid plastic material into the
second cavity to produce the handle. The handle, after cooling, is
pulled away from the turret face at the third or at a fourth
station for separating the bristles from the first cavity at the
controlled rate referred to, advantageously after a partial
detachment of the bristles from their cell walls by a limited
separation of two members jointly defining the cells of the first
cavity.
BRIEF DESCRIPTION OF THE DRAWING
The above and other features of my invention will now be described
in detail with reference to the accompanying drawing in which:
FIGS. 1A and 1B respectively show an elevation and an end view of a
composite brush molded in accordance with the method of this
invention;
FIGS. 2A and 2B respectively show an elevation and an end view of a
different brush molded in accordance with this method;
FIG. 3 is a perspective view of a third form of brush molded in
accordance with my invention;
FIG. 4 is a fragmentary cross-sectional view taken on the line 4--4
of FIG. 3;
FIGS. 5 and 6 are plan views of the significant components of a
molding machine shown in two sequential stage of operation in
accordance with this invention;
FIGS. 7 and 8 are similar to FIGS. 5 and 6, showing a modification
of the molding machine operated according to this invention;
FIG. 9 is a timing diagram pertaining to the operation of the
machine of FIGS. 5 and 6; and
FIG. 10 is a timing diagram pertaining to the operation of FIGS. 7
and 8.
DETAILED DESCRIPTION
In my early experiments with Kraton, a fairly heavy backing in the
shape of a band was molded integral with the bristles. The mold was
then opened whereupon the bristles were gradually extracted from
their cavity cells manually by pulling slowly on the band. Because
of the thickness of the band, it had a rather stiff structure and
thus could act as a relatively solid base for successively pulling
the bristles out by hand. Mold-release spray was found to help the
disengagement of the bristles, but the use of the spray added more
time to an already slow operation. Furthermore, a major
disadvantage of using mold-release sprays is the danger of
contamination, particularly for brushes which are to be used in
surgical applications.
Because of the thickness of the band just mentioned, on the order
of 3 mm, the band not only added a considerable amount of weight to
the workpiece but also was very slow in cooling. It will be evident
that the heavy weight of the band, the slow cooling cycle and the
slow method of removal would have made for an expensive artidle if
this line of development had been pursued.
In lieu of the integral brush structure just mentioned, the method
according to my invention is adapted to produce a two-material
brush of the kind shown in FIGS. 1A-4. In FIGS. 1A and 1B a brush
10 is seen to include a multiplicity of bristles 11, of Kraton or
comparable flexible material, and a handle 12 made of any fairly
stiff, compatible and low-cost material such as polyethylene.
Polyethylene is quite suitable for this application because it is
capable of bonding with the Kraton under heat and pressure from the
injection. If other materials are to be used, they may require a
bonding agent.
It will be noted that the handle of the brush shown in FIGS. 1A and
1B is shaped like an upwardly open box with a slight undercut 13
around the inside edge or lip. This undercut allows the handle 12
to remain in gripping contact with the mold portions defining the
cavity in which it is formed, so that retraction of those mold
portions will pull on the handle 12 and thus also on the bristles
11. Once the bristles are removed from the cells of their cavity,
the handle 12 can be separated from the corresponding mold part
with the aid of standard ejector pins.
In FIGS. 2A and 2B a similar brush 10a is shown wherein, however,
the handle 12a is essentially in the form of a closed loop
extending upward at right angles to a backing layer 14 adjoining
the bristles 11. The handle 12a also includes a relatively stiff
plate portion 15 bonded to the backing layer 14. The mold parts
defining the handle cavity may separate along the parting line 16,
or else side cores withdraw after the bristles have been pulled out
of their cells and the mold parts can then fall out. As an
alternative, the bristles may stay in their cells and then in the
next station be slowly extracted by the action of a take-off member
gripping the handle.
FIGS. 3 and 4 show another way of retaining a brush handle in its
cavity in cases where the construction shown in the preceding
Figures cannot be used. Undercuts 17 in the flat portion 18 of the
handle 12b are defined by small "buttons" or bosses 19 which create
a dovetail effect and assure that enough force is available to pull
the bristles out of their cavity cells. Ejector pins 20 aligned
with the buttons 19 are then used to discharge the finished
brush.
The molds of FIGS. 5-8, now to be discussed in detail, include
bristle-cavity cells formed by sliding blades as disclosed in my
above-identified prior U.S. Pat. Nos. 3,004,291 and 3,128,488.
However, nonseparating unitary cavity structures would operate
similarly and would not require the hydraulic plate-separation
mechanism and controls within a turret which will be described
hereinafter.
For the practice of my novel method, a machine such as that
disclosed in U.S. Pat. No. 3,454,991 may be used. Generally, I may
utilize any molding machine equipped with two injection systems and
a transfer mechanism to carry the unfinished parts (bristles) from
a first to a second injection station where the handle can be
molded onto them. The preferred machine operable according to this
method comprises a four-station turret as disclosed in U.S. Pat.
No. 4,330,257.
Attention is first directed to FIGS. 5 and 6, to be viewed in
conjunction with the timing diagram shown in FIG. 9.
In FIGS. 5 and 6, a base 110 represents a two-shot molding machine
equipped with two injection systems 111 and 112, as described in
U.S. Pat. Nos. 4,243,362 and 4,330,257. This machine has a clamping
system consisting of a stationary platen 114, a moving platen 116
and a turret 118. A clamp column 120 drives the platen 116 to open
or close the molds. The turret 118, supported in a nonillustrated
manner on the base 110 and guided on tie bars 22, is moved in the
direction of the clamp column 120--but at a slower speed--by a
mechanism which is not shown. The latter motion is sufficient to
permit rotation of the turret 118 during mold-open time, without
interfering with the mold portions on the stationary and moving
platens. The open position is illustrated in FIG. 6. This machine
will be seen to produce brushes 10 as described above with
reference to FIGS. 1A and 1B.
The stationary platen 114 carries a fixed mold half 30 consisting
of a cavity plate 32 and a backing plate 34, with a hot-runner
system 36 supported therebetween in minimal contact with members 32
and 34. The hot-runner system supplies plastic through heated ducts
38 from the injection unit 111 to a cavity structure 40 defining a
relatively narrow, flat clearance 42 in mold half 30 which forms
the thin common backing layer 14 of the preceding Figures, and
which faces elongate bristle-forming cells 44 that are part of a
core section 50 on a confronting face of turret 118. There are four
identical core sections 50, one for each of the four sides of the
turret. Each such core section 50 consists of a front plate 52, a
backing plate 54, and for each hot-runner branch a blade pack which
defines the elongate cavity cells 44. Each blade pack consists of
one set of blades 56 anchored in the backing plate 54 and a
matching set of interfitting blades 58 held in front plate 52 (best
seen in FIG. 6).
Guide pins 60 slidably connect the front plates 52 with the
associated backing plates 54 and line up the core sections 50 with
the mold half 30 on the stationary platen 114 and with another mold
half 80 on the moving platen 116. Core sections 50 and mold halves
30, 80 respectively constitute the primary, secondary and tertiary
mold portions referred to above.
Water-cooling channels 62,64 and 66 are shown in plates 32,52 and
54, respectively. All water channels in the mold sections 50
mounted on the turret are fed from a central supply through the
shaft of the turret (not shown).
Hydraulic cylinders 70 in the turret are located under each core
section 50, and a piston rod 72 is mounted on each plate 52 so
that, when the cylinder 70 is pressurized at a port 74, the plate
52 is pulled against the plate 54. Conversely, when the cylinder 70
is pressurized at a port 76, the plate 52 is pushed forward, thus
withdrawing the blades 56 from the blades 58 and thereby releasing
the molded bristles, as disclosed in U.S. Pat. No. 3,004,291.
The handle-forming mold half 80 mounted on the moving platen 116
consists of a cavity plate 82, cavity inserts 84 serving as
handle-gripping formations, a hot-runner intermediate plate 86 and
a backing plate 88.
A hot-runner block 90 is supported with minimum contact between
plates 82 and 86. Plastic is ducted through channels 91 from the
injection system 112 to cavities 92 (see FIG. 6).
Ejector pins 94 are carried on a shiftable plate 95 and pass
through plates 86, 88 and 82. The plate 95 is actuated by the
ejection system of the molding machine, not further illustrated,
through pins 96. The plate 82 and cavity inserts 84 are
water-cooled through appropriate channels 98.
The operation of the machine of FIGS. 5 and 6 is now to be
described; reference may simultaneously be had to the timing
diagram of FIG. 9.
As the mold closes, the bristles and their thin backing layer are
molded in cavity structure 40, in position 1 (to the right in FIG.
5), from a relatively flexible plastic introduced from the
injection unit 111 through channel 38.
As the mold opens (FIG. 6) the bristles with their backing layer
remain in the core section or primary mold portion A. The turret
now turns through 90.degree. in a clockwise direction, as viewed in
the drawing, and the just-molded bristles move into position 2
(down in FIGS. 5 and 6). The bristles which had previously been
molded and held in core section B now move into position 3 (to the
left in FIGS. 5 and 6), thus facing the tertiary mold portion 80.
The mold now recloses, and the previously empty core section D now
faces the secondary mold portion 30 for filling with plastic from
injection unit 111.
Injections from units 111 and 112 now take place simultaneously. In
position 1 a new bristle portion is molded, and in position 3 the
handle is molded onto the previously molded bristles which are
still held in their cavity cells for continuing cooling.
As soon as the handle is ready for ejection (in the timing diagram
of FIG. 9 this is tentatively indicated as 15 seconds), the mold
opens. It is now very important that this opening stroke proceed
very slowly in the first phase of the platen motion. The hydraulic
cylinder 70 is pressurized as soon as the mold opens so that plate
52 follows the receding mold half 80. Blades 56 slide out from
between the blades 58 interleaved therewith to release the
bristles. It should be noted that even though the bristles have now
ample space to withdraw, their plastic material is very sticky and
tends to cling to the blades 58 of the outer plate 52. In order to
give the bristles time to detach themselves slowly from the latter
blades, the pulling-out motion must be very gradual; otherwise the
bristles can tear and remain stuck in their cells.
The slow opening is indicated in the timing diagram of FIG. 9 by
allowing 3 seconds rather than the normally required 1 second for
the opening stroke. As soon as the mold is opened far enough, the
machine ejectors 96 are activated and push the sliding plate 95
forward so that the pins 94 dislodge the finished brushes from the
cavities. By reversing the force on piston 71, the plate 52 is
returned to sit again on plate 54. The turret now turns through
another 90.degree. in a clockwise direction, as seen in the
drawing, whereupon the empty core section C is in position 4,
either simply waiting or (if permissible) being sprayed from an
automatic mold-release dispenser (not shown) which is in common
usage and commercially available.
It can be readily seen that, with the sequence described above, the
bristles remain in their cavities more than three times the term
allowed for the injection of the handle, during which period they
are subjected to cooling by the fluid circulating through channels
64 and 66.
The diagram shown in FIG. 9 illustrates by way of example an
assumed time sequence with the injection and cooling time for the
handle taken to be 15 seconds. The diagram also assumes closing,
turning and opening times of 1, 1 and 3 seconds, respectively. The
time during which the bristles are subjected to cooling is almost
four times as long as the injection and cooling time for the
handle.
It is to be noted that the intervals shown in the timing diagram of
FIG. 9 are merely assumed and could be more or less, depending on
product design and mold construction. However, the diagram makes it
clear that the time during which the bristles are exposed to
cooling is 3 to 4 times as long as the molding cycle which is
limited by the injection and cooling time of the handle.
A different and advantageous sequence is illustrated in FIGS. 7 and
8 and attention is now directed to these Figures along with the
timing diagram shown in FIG. 10.
In the machine shown in FIGS. 7 and 8, the mold structure and the
method of injection are substantially the same as those described
with reference to FIGS. 5 and 6 except that the brushes to be
produced are of the type shown at 10a in FIGS. 2A and 2B. Also,
position 3 adjacent platen 116 is used only to inject the handle
onto the bristles, and not to withdraw the bristles from their
cavity cells. The extraction of the bristles takes place at a
fourth station occupied in position 4 (up in FIGS. 7 and 8). At
that station, a take-off mechanism 130 is withdrawable to a waiting
position (shown in FIG. 8) in which it is far enough back so as not
to impede the rotation of the turret 118. As the mold opens after
cooling of the handle in position 3, each newly formed handle stays
with the bristles on the core section 50. The turret now turns
through 90.degree. in the clockwise direction, as seen in the
drawing, and the clamp recloses for the next cycle. The brushes
last produced now face the take-off mechanism 130, which advances
and engages the loop-shaped handles with suitable gripping
formations constituted by claws 132 that merely project at right
angles from their supports and constitute retractors designed to
pass into the openings 134 of the loops. Of course, many other
engagement and retractor designs could be utilized.
The cylinder 70 is now pressurized very slowly to separate the
plates 52 and 54 at a gradual pace, thereby slowly freeing the
bristles from their cells. The retractor assembly constituting the
take-off device 130 is so controlled that the initial motion is
very slow for the first 10 mm or so, before the retraction is
accelerated to deliver the brushes to their drop-off point.
Thereafter, the mechanism 130 returns to the waiting position shown
in FIG. 8.
Two main advantages become evident when comparing the sequence
shown in FIGS. 7 and 8 with that described with respect to FIGS. 5
and 6. Firstly, the separation of the platens can take place at
full speed, as is illustrated in the timing diagram of FIG. 10, by
allowing only 1 second for the opening. Assuming the same injection
and cooling time for the handle as previously (15 seconds), the
total cycle is now reduced from 20 to 18 seconds, almost without
affecting the time allowed for the bristles to cool (54 seconds
instead of 55 seconds), or almost four times the injection and
cooling time of the handle.
Secondly, a good portion of the 15 seconds during which the mold is
closed is now available for the withdrawal of the bristles.
Assuming times for the action of the take-off device
(in-grab-out-release-reset) of about 8 seconds, there are still 7
seconds available for the slow pull-out of bristles over the first
few millimeters. This advantage would be even greater if the time
required for slow withdrawal in the sequence of FIG. 9 were more
than 3 seconds.
It is again stressed that the times shown are given only by way of
illustration and could be more or less, according to part design
and mold construction. However, in essence the cooling time of the
bristles is three to four times the injection and cooling time of
the handle.
It will be appreciated that the molding machine shown could include
an attachment making it possible to apply a bonding material to the
thin backing layer in position 2, in such cases where the material
of the handle is not readily bonded by heat and pressure to the
bristle material. Such setup would add to the cost of the
installation, but would not affect the molding cycle. Conversely,
the exposed major surface of the backing layer could be provided
with protuberances having undercuts around which the plastic
material of the handle would harden, thereby providing a mechanical
bonding between the bristles and the handle in cases where the two
materials do not automatically adhere to each other.
My invention is also applicable to the production of a pre-soaked
disposable brush. It will be readily appreciated that, after
ejection of the brush in either station 3 or station 4, a jet of
liquid or powdered soap can be directed into the bristle cavities
at station 4, in a manner similar to the conventional
mold-release-spraying method. The soap will adhere lightly to the
walls of the cavities and will not impede the filling thereof
during injection in station 1. The soap then will coat the sticky
plastic of the bristles during ejection. In the use of the brush,
the bristles are simply dipped into water and the soap film will
dissolve to produce a ready foam.
* * * * *